FIGS. 1(a) and 1(b) schematically show one example of ceramic honeycomb filters for capturing particulate matter in exhaust gases from automobiles. The ceramic honeycomb filter 11 comprises a ceramic honeycomb structure 1 having large numbers of flow paths 3, 4 partitioned by porous cell walls 2 extending from an inlet end surface 12 to an outlet end surface 13, and plugs 5, 5 for sealing flow paths 3, 4 alternately in a checkerboard pattern on both end surfaces 12, 13. The porous cell walls 2 have as high porosity as, for instance, 55-80% to avoid pressure loss increase, because they carry catalytic materials for burning particulate matter in the exhaust gas at low temperatures.
As a method for producing a ceramic honeycomb filter having such structure, JP2001-300922A and JP2002-28915A disclose, as shown in FIG. 2, a method comprising the steps of (a) attaching a film 6 to an end surface 12 of a ceramic honeycomb structure 1, (b) providing the film 6 with holes 7 at positions corresponding to flow paths 3, (c) placing the ceramic honeycomb structure 1 in a plugging material slurry 8 containing ceramic particles, an organic binder, water, etc. with the end surface 12 downward, (d) introducing the plugging material slurry 8 into the flow paths 3 through the holes 7 of the film 6 while pressing the ceramic honeycomb structure 1 downward by a pressing means 10, (e) dewatering and hardening the plugging material slurry 8 entering into the flow paths 3, and finally (f) peeling the film 6.
However, because the porous cell walls 2 are, for instance, as thin as 0.1-0.5 mm and have as high porosity as 55-80%, they easily have chipped portions 21 on the end surface 12 as shown in FIG. 3. Also, because there is no sufficient adhesion between the end surfaces 12 of the thin porous cell walls 2 and the film 6, gaps are likely to be generated between the end surfaces 12 and the film 6 by pressing. As a result, the plugging material slurry 8 is likely to leak to adjacent flow paths 4 that should not be plugged at the end surface 12, and stick to their cell walls 2 as protrusions 51 as schematically shown in FIG. 4. The formation of the plugging material protrusions 51 occurs on both sides of the ceramic honeycomb structure 1. With the plugging material protrusions 51 formed, the flow paths 3, 4 are narrowed, resulting in pressure loss increase in the ceramic honeycomb filter 11. In addition, particulate matter in the exhaust gas is accumulated on the plugging material protrusions 51, pressure loss increases in the ceramic honeycomb filter 11 in a short period of time.
JP9-29019A discloses a method for plugging a ceramic honeycomb structure, which comprises (a) covering an end surface of a ceramic honeycomb structure with a film, (b) providing the film with holes at positions corresponding to flow paths that should not be plugged, (c) charging a temporary plugging material slurry comprising powder insoluble in a solvent for a plugging material slurry and burnable or decomposable by heating, into the flow paths through the holes, (d) peeling the film from the end surface after the temporary plugging material slurry is dried, (e) charging the plugging material slurry into the end portions of flow paths to be plugged, and (f) sintering the plugging material slurry while decomposing the resultant temporary plugs. Because the plugging material slurry is charged into the flow paths to be plugged after the temporary plugging material slurry is charged into the flow paths that should not be plugged, the plugging material slurry does not leak to the flow paths that should not be plugged. However, with the temporary plugging material slurry leaking to the flow paths to be plugged, the flow paths to be plugged are insufficiently filled with the plugging material slurry, and the plugs are bonded to the cell walls with insufficient adhesion strength. In addition, this method is not efficient because it needs two slurry-charging steps.